Rotary pump with hydrodynamically suspended impeller

ABSTRACT

This invention relates to rotary pumps adapted, but not exclusively, for use as artificial hearts or ventricular assist devices and, in particular, discloses in preferred forms a seal-less shaft-less pump featuring open or closed (shrouded) impeller blades with the edges of the blades used as hydrodynamic thrust bearings and with electromagnetic torque provided by the interaction between magnets embedded in the blades and a rotating current pattern generated in coils fixed relative to the pump housing.

FIELD OF THE INVENTION

This invention relates to rotary pumps adapted, but not exclusively, foruse as artificial hearts or ventricular assist devices and, inparticular, discloses in preferred forms a seal-less shaft-less pumpfeaturing open or closed (shrouded) impeller blades with the edges ofthe blades used as hydrodynamic thrust bearings and with electromagnetictorque provided by the interaction between magnets embedded in theblades and a rotating current pattern generated in coils fixed relativeto the pump housing.

BACKGROUND ART

This invention relates to the art of continuous or pulsatile flow rotarypumps and, in particular, to electrically driven pumps suitable for usealthough not exclusively as an artificial heart or ventricular assistdevice. For permanent implantation in a human patient, such pumps shouldideally have the following characteristics: no leakage of fluids into orfrom the bloodstream; parts exposed to minimal or no wear; minimumresidence time of blood in pump to avoid thrombosis (clotting); minimumshear stress on blood to avoid blood cell damage such as haemolysis;maximum efficiency to maximise battery duration and minimise bloodheating; and absolute reliability.

Several of these characteristics are very difficult to meet in aconventional pump configuration including a seal, i.e. with an impellermounted on a shaft which penetrates a wall of the pumping cavity, asexemplified by the blood pumps referred to in U.S. Pat. No. 3,957,389 toRafferty et al., U.S. Pat. No. 4,625,712 to Wampler, and U.S. Pat. No.5,275,580 to Yamazaki. Two main disadvantages of such pumps are firstlythat the seal needed on the shaft may leak, especially after wear, andsecondly that the rotor of the motor providing the shaft torque remainsto be supported, with mechanical bearings such as ball-bearingsprecluded due to wear. Some designs, such as U.S. Pat. No. 4,625,712 toWampler and U.S. Pat. No. 4,908,012 to Moise et al., have overcome theseproblems simultaneously by combining the seal and the bearing into onehydrodynamic bearing, but in order to prevent long residence times theyhave had to introduce means to continuously supply a blood-compatiblebearing purge fluid via a percutaneous tube.

In seal-less designs, blood is permitted to flow through the gap in themotor, which is usually of the brushless DC type, i.e. comprising arotor including permanent magnets and a stator in which an electriccurrent pattern is made to rotate synchronously with the rotor. Suchdesigns can be classified according to the means by which the rotor issuspended: contact bearings, magnetic bearings or hydrodynamic bearings,though some designs use two of these means.

Contact or pivot bearings, as exemplified by U.S. Pat. No. 5,527,159 toBozeman et al. and U.S. Pat. No. 5,399,Q74 to Nos6 et al., havepotential problems due to wear, and cause very high localised heatingand shearing of the blood, which can cause deposition and denaturationof plasma proteins, with the risk of embolisation and bearing seizure.

Magnetic bearings, as exemplified by U.S. Pat. No. 5,350,283 to Nakazekiet al., U.S. Pat. No. 5,326,344 to Bramm et al. and U.S. Pat. No.4,779,614 to Moise et al., offer contactless suspension, but requirerotor position measurement and active control of electric current forstabilisation of the position in at least one direction, according toEarnshaw's theorem. Position measurement and feedback control introducesignificant complexity, increasing the failure risk. Power use by thecontrol current implies reduced overall efficiency. Furthermore, size,mass, component count and cost are all increased.

U.S. Pat. No. 5,507,629 to Jarvik claims to have found a configurationcircumventing Earnshaw's Theorem and thus requiring only passivemagnetic bearings, but this is doubtful and contact axial bearings areincluded in any case. Similarly, passive radial magnetic bearings and apivot point are employed in U.S. Pat. No. 5,443,503 to Yamane.

Prior to the present invention, pumps employing hydrodynamic suspension,such as U.S. Pat. No. 5,211,546 to Isaacson et al. and U.S. Pat. No.5,324,177 to Golding et al., have used journal bearings, in which radialsuspension is provided by the fluid motion between two cylinders inrelative rotation, an inner cylinder lying within and slightly off axisto a slightly larger diameter outer cylinder. Axial suspension isprovided magnetically in U.S. Pat. No. 5,324,177 and by either a contactbearing or a hydrodynamic thrust bearing in U.S. Pat. No. 5,211,546.

A purging flow is needed through the journal bearing, a high shearregion, in order to remove dissipated heat and to prevent long fluidresidence time. It would be inefficient to pass all the fluid throughthe bearing gap, of small cross-sectional area, as this would demand anexcessive pressure drop across the bearing. Instead a leakage path isgenerally provided from the high pressure pump outlet, through thebearings and back to the low pressure pump inlet, implying a smallreduction in outflow and pumping efficiency. U.S. Pat. No. 5,324,177provides a combination of additional means to increase the purge flow,namely helical grooves in one of the bearing surfaces, and a smalladditional set of impellers.

U.S. Pat. No. 5,211,546 provides 10 embodiments with various locationsof cylindrical bearing surfaces. One of these embodiments, the third,features a single journal bearing and a contact axial bearing.

Embodiments of the present invention offer a relatively low cost and/orrelatively low complexity means of suspending the rotor of a seal-lessblood pump, thereby overcoming or ameliorating the problems of existingdevices mentioned above.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is disclosed arotary blood pump with impeller suspended hydrodynamically by thrustforces generated on the edges of the impeller blades. The blade edgesare shaped such that the gap between the edges and the housing at theleading edge is greater than at the trailing edge and thus the fluidwhich is drawn through the gap experiences a wedge shaped restrictionwhich generates a thrust away from the housing, as described inReynold's theory of lubrication.

In preferred embodiments of the invention, the pump is of centrifugal ormixed flow type with impeller blades open on both the front and backfaces of the housing. At least one face of the housing is made conical,in order that the thrust perpendicular to it has a radial component,which provides a radial restoring force to a radial displacement of theimpeller axis. Similarly, an axial displacement toward either the frontor the back face increases the thrust from that face and reduces thethrust from the other face. Thus the sum of the forces on the impellerdue to inertia (within limits), gravity and any bulk radial or axialhydrodynamic force on the impeller can be countered by a restoring forcefrom the thrust bearings after a small displacement of the impellerwithin the housing relative to the housing in either a radial or axialdirection.

In the preferred embodiment, the impeller driving torque derives fromthe magnetic interaction between permanent magnets within the blades ofthe impeller and oscillating currents in windings encapsulated in thepump housing.

In a second embodiment of the invention, the principle is applied in apump of axial type. Within a uniform cylindrical section of the pumphousing, tapered blade edges form a radial hydrodynamic bearing. If thepump housing is made with reducing radius at the two ends, then the endhydrodynamic thrust forces have an axial component which can provide theaxial bearing. Alternatively, magnetic forces or other means can providethe axial bearing.

In a further broad form of the invention there is provided a rotaryblood pump having an impeller suspended hydrodynamically by thrustforces generated by the impeller during movement in use of the impeller.

Preferably said thrust forces are generated by blades of said impelleror by deformities therein.

More preferably said thrust forces are generated by edges of said bladesof said impeller.

Preferably said edges of said blades are tapered.

In an alternative preferred form said pump is of axial type.

Preferably within a uniform cylindrical section of the pump housing,tapered blade edges form a radial hydrodynamic bearing.

Preferably the pump housing is made with reducing radius at the twoends, and wherein the end hydrodynamic thrust forces have an axialcomponent which can provide the axial bearing.

Preferably or alternatively magnetic forces or other means can providethe axial bearing.

In a further broad form of the invention there is provided a rotaryblood pump having a housing within which an impeller acts by rotationabout an axis to cause a pressure differential between an inlet side ofa housing of said pump and an outlet side of the housing of said pump;said impeller suspended hydrodynamically by thrust forces generated bythe impeller during movement in use of the impeller.

BRIEF DESCRIPTION of THE DRAWINGS

Embodiments of the present invention will now be described, withreference to the accompanying drawings, wherein:

FIG. 1 is a longitudinal cross-sectional view of a preferred embodimentof the invention;

FIG. 2 is a cross-sectional view taken generally along the line Z-Z ofFIG. 1;

FIG. 3A is a cross-sectional view of an impeller blade taken generallyalong the line A-A of FIG. 2;

FIG. 3B is an enlargement of the blade-pump housing interface portion ofFIG. 3A;

FIG. 3C is an alternative impeller blade shape;

FIGS. 4A, B, C illustrate various possible locations of magnet materialwithin a blade;

FIG. 5 is a left-hand end view of a possible winding geometry takengenerally along the line S-S of FIG. 1;

FIG. 6 is a diagrammatic cross-sectional view of an alternativeembodiment of the invention as an axial pump;

FIG. 7 is an exploded, perspective view of a centrifugal pump assemblyaccording to a further embodiment of the invention;

FIG. 8 is a perspective view of the impeller of the assembly of FIG. 7;

FIG. 9 is a perspective, cut away view of the impeller of FIG. 8 withinthe pump assembly of FIG. 7;

FIG. 10 is a side section indicative view of the impeller of FIG. 8;

FIG. 11 is a detailed view in side section of edge portions of theimpeller of FIG. 10;

FIG. 12 is a block diagram of an electronic driver circuit for the pumpassembly of FIG. 7;

FIG. 13 is a graph of head versus flow for the pump assembly of FIG. 7;

FIG. 14 is a graph of pump efficiency versus flow for the pump assemblyof FIG. 7;

FIG. 15 is a graph of electrical power consumption versus flow for thepump assembly of FIG. 7;

FIG. 16 is a plan, section view of the pump assembly showing a volutearrangement according to a preferred embodiment;

FIG. 17 is a plan, section view of a pump assembly showing analternative volute arrangement;

FIG. 18 is a plan view of an impeller according to a further embodimentof the invention;

FIG. 19 is a plan view of an impeller according to a further embodimentof the invention;

FIG. 20 is a perspective view of an impeller according to a furtherembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The pump assemblies according to various preferred embodiments to bedescribed below all have particular, although not exclusive, applicationfor implantation in a mammalian body so as to at least assist, if nottake over, the function of the mammalian heart. In practice this isperformed by placing the pump assembly entirely within the body of themammal and connecting the pump between the left ventricle and the aortaso as to assist left side heart function. It may also be connected tothe right ventricle and pulmonary artery to assist the right side of theheart.

In this instance the pump assembly includes an impeller which is fullysealed within the pump body and so does not require a shaft extendingthrough the pump body to support it. The impeller is suspended, in use,within the pump body by, at least, the operation of hydrodynamic forcesimparted as a result of the interaction between the rotating impeller,the internal pump walls and the fluid which the impeller causes to beurged from an inlet of the pump assembly to an outlet thereof.

A preferred embodiment of the invention is the centrifugal pump 1, asdepicted in FIGS. 1 and 2, intended for implantation into a human body,in which case the fluid referred to below is blood. The pump housing 2,can be fabricated in two parts, a front part 3 in the form of a housingbody and a back part 4 in the form of a housing cover, with a smoothjoin therebetween, for example at 5 in FIG. 1. The pump 1 has an axialinlet 6 and a tangential outlet 7. The rotating part or impeller 100 isof very simple form, comprising only blades 8 and a blade support 9 tohold those blades fixed relative to each other. The blades may be curvedas depicted in FIG. 2, or straight, in which case they can be eitherradial or tilted, i.e. at an angle to the radius. This rotating part 100will hereafter be called the impeller 100, but it also serves as abearing component and as the rotor of a motor configuration as to befurther described below whereby a torque is applied by electromagneticmeans to the impeller 100. Note that the impeller has no shaft and thatthe fluid enters the impeller from the region of its axis RR. Some ofthe fluid passes in front of the support cone 9 and some behind it, sothat the pump 1 can be considered of two-sided open type, as compared toconventional open centrifugal pumps, which are only open on the frontside. Approximate dimensions found adequate for the pump 1 to perform asa ventricular assist device, when operating at speeds in the range 2,000rpm to 4,000 rpm, are outer blade diameter 40 mm, outer housing averagediameter 60 mm, and housing axial length 40 mm.

As the blades 8 move within the housing, some of the fluid passesthrough the gaps, much exaggerated in FIGS. 1 and 3, between the bladeedges 101 and the housing front face 10 and housing back face 11. In allopen centrifugal pumps, the gaps are made small because this leakageflow lowers the pump hydrodynamic efficiency. In the pump disclosed inthis embodiment, the gaps are made slightly smaller than is conventionalin order that the leakage flow can be utilised to create a hydrodynamicbearing. For the hydrodynamic forces to be sufficient, the blades mustalso be tapered as depicted in FIGS. 3A and 3B, so that the gap 104 islarger at the leading edge 102 of the blade 8 than at the trailing edge103. The fluid 105 which passes through the gap thus experiences a wedgeshaped restriction which generates a thrust, as described in Reynolds'theory of lubrication (see, for example, “Modern Fluid Dynamics, Vol. 1Incompressible Flow”, by N. Curle and H. J. Davies, Van Nostrand, 1968).The thrust is proportional to the square of the blade thickness at theedge, and thus thick blades are favoured, since if the proportion of thepump cavity filled by blades is constant, then the net thrust force willbe inversely proportional to the number of blades. However, the bladeedges can be made to extend as tails from thin blades as depicted inFIG. 3C in order to increase the blade area adjacent the walls.

In one particular form, the tails join adjacent blades so as to form acomplete shroud with wedges or tapers incorporated therein. An exampleof a shroud design as well as other variations on the blade structurewill be described later in this specification.

For manufacturing simplicity, the housing front face 10 can be madeconical, with an angle of around 450 so that it provides both axial andradial hydrodynamic forces. Other angles are suitable that achieve thefunctional requirements of this pump including the requirements for bothaxial and radial hydrodynamic forces.

Other curved surfaces are possible provided both axial and radialhydrodynamic forces can be produced as a result of rotation of theblades relative to the housing surfaces.

The housing back face 11 can include a roughly conical extension 12pointing into the pump cavity 106, to eliminate or minimise the effectof the flow stagnation point on the axis of the back housing.

Alternatively extension 12 can resemble an impeller eye to make the flowmixed.

In this preferred embodiment, for manufacturing simplicity and foruniformity in the flow axial direction RR, the housing back face 11 ismade flat over the bearing surfaces, i.e. under the blade edges. Withthis the case, a slacker tolerance on the alignment between the axes ofthe front part 3 and back part 4 of the housing 2 is permissible. Analternative is to make the back face 11 conical at the bearing surfaces,with taper in the opposite direction to the front face 10, so that thehydrodynamic forces from the back face will also have radial components.Tighter tolerance on the axes alignment would then be required, and someof the flow would have to undergo a reversal in its axial direction.Again a roughly conical extension (like 12) will be needed. There may besome advantage in making the housing surfaces and blade edgesnon-straight, with varying tangent angle, although this will imposegreater manufacturing complexity.

There are several options for the shape of the taper, but in thepreferred embodiment the amount of material removed simply varieslinearly or approximately linearly across the blade. For the back face,the resulting blade edges are then planes at a slight inclination to theback face. For the front face, the initial blade edges are curved andthe taper only removes a relatively small amount of material so theystill appear curved. Alternative taper shapes can include a step in theblade edge, though the corner in that step would represent a stagnationline posing a thrombosis risk.

For a given minimum gap, at the trailing blade edge, the hydrodynamicforce is maximal if the gap at the leading edge is approximately doublethat at the trailing edge. Thus the taper, which equals the leading edgegap minus the trailing edge gap, should be chosen to match a nominalminimum gap, once the impeller has shifted towards that edge. Dimensionswhich have been found to give adequate thrust forces are a tape r ofaround 0.05 mm for a nominal minimum gap of around 0.05 mm, and anaverage circumferential blade edge thickness of around 5 mm for 4blades. For the front face, the taper is measured within the planeperpendicular to the axis. The axial length of the housing between thefront and back faces at any position should then be made about 0.2 mmgreater than the axial length of the blade, when it is coaxial with thehousing, so that the minimum gaps are both about 0.1 mm axially when theimpeller 100 is centrally positioned within the housing 2. Then, forexample, if the impeller shifts axially by 0.05 mm, the minimum gapswill be 0.05 mm at one face and 0.15 mm at the other face. The thrustincreases with decreasing gap and would be much larger from the 0.05 mmgap than from the 0.15 mm gap, about 14 times larger for the abovedimensions. Thus there is a net restoring force away from the smallergap.

Similarly, for radial shifts of the impeller the radial component of thethrust from the smaller gap on the conical housing front face wouldoffer the required restoring radial force. The axial component of thatforce and its torque on the impeller would have to be balanced by anaxial force and torque from the housing back face, and so the impellerwill also have to shift axially and tilt its axis to be no longerparallel with the housing axis. Thus as the person moves and the pump isaccelerated by external forces, the impeller will continually shift itsposition and alignment, varying the gaps in such a way that the totalforce and torque on the impeller 100 match that demanded by inertia. Thegaps are so small, however, that the variation in hydrodynamicefficiency will be small, and the pumping action of the blades will beapproximately the same as when the impeller is centrally located.

While smaller gaps imply greater hydrodynamic efficiency and greaterbearing thrust forces, smaller gaps also demand tighter manufacturingtolerances, increase frictional drag on the impeller, and impose greatershear stress an the fluid. Taking these points in turn, for the above0.05 mm tapers and gaps, tolerances of around ±0.015 mm are needed,which imposes some cost penalty but is achievable. A tighter toleranceis difficult, especially if the housing is made of a plastic, given thechanges in dimension caused by temperature and possible absorption offluid by plastic. The frictional drag for the above gaps produces muchsmaller torque than the typical motor torque. Finally, to estimate theshear stress, consider a rotation speed of 3,000 rpm and a typicalradius of 15 mm, at which the blade speed is 4.7 ms-¹ and the averagevelocity shear for an average gap of 0.075 mm is 6.2×10⁴ s⁻¹. For bloodof dynamic viscosity 3.5×10⁻³ kgm-¹s-¹, the average shear stress wouldbe 220 Nm⁻². Other prototype centrifugal blood pumps with closed bladeshave found that slightly larger gaps, e.g. 0.15 mm, are acceptable forhaemolysis. A major advantage of the open blades of the presentinvention is that a fluid element that does pass through a blade edgegap will have very short residence time in that gap, around 2×10⁻³ S,and the fluid element will most likely be swept though the pump withoutpassing another blade edge.

To minimise the net force required of the hydrodynamic bearings, the netaxial and radial hydrodynamic forces on the impeller from the bulk fluidflow should be minimised, where “bulk” here means other than from thebearing thrust surfaces.

One method of minimising the bulk radial hydrodynamic force is to usestraight radial blades so that pressure acting on the blade sides hasvirtually no radial component. The radial force on the impeller dependscritically on the shape of the output flow collector or volute 13. Theshape should be designed to minimise the radial impeller force over thedesired range of pump speeds, without excessively lowering the pumpefficiency. The optimal shape will have a roughly helical perimeterbetween the “cut water” and outlet. The radial force can also be reducedby the introduction of an internal division in the volute 13 to create asecond output flow collector passage, with tongue approximatelydiametrically opposite to the tongue of the first passage.

An indicative plan view of impeller 100 relative to housing 2 is shownin FIG. 2 having a concentric volute 13.

FIG. 17 illustrates the alternative volute arrangement comprising asplit volute created by volute barrier 107 which causes volute 108 in afirst hemisphere of the housing 2 to split into first half volute 109and second half volute 110 over the second hemisphere. The hemispheresare defined respectively on each side of a diameter of the housing 2which passes through or near exit point 111 of outlet 7.

In alternative forms concentric volutes can be utilised, particularlywhere specific speed is relatively low.

In a further particular form a vaneless diffuser may also reduce theradial force.

In regard to the bulk axial hydrodynamic axial force, if the bladecross-section is made uniform in the axial direction along therelational axis, apart from the conical front edge, then the pressureacting on the blade surface (excluding the bearing edges) will have noaxial component. This also simplifies the blade manufacture. The bladesupport cone 9 must then be shaped to minimise axial thrust on theimpeller and minimise disturbance to the flow over the range of speeds,while maintaining sufficient strength to prevent relative blademovement. The key design parameter affecting the axial force is theangle of the cone. The cone is drawn in FIG. 1 as having the sameinternal diameter as the blades, which may aid manufacture. However, thecone could be made with larger or smaller internal diameter to theblades. There may be advantage in using a non-axisymmetric support“cone”, e.g. with larger radius on the trailing surface of a blade thanthe radius at the leading surface of the next blade. If the blades aremade with non-uniform cross-section to increase hydrodynamic efficiency,then any bulk hydrodynamic axial force on them can be balanced byshaping the support cone to produce an opposite bulk hydrodynamic axialforce on it.

Careful design of the entire pump, employing computational fluiddynamics, is necessary to determine the optimal shapes of the blades 8,the volute 13, the support cone 9 and the housing 2, in order tomaximise hydrodynamic efficiency while keeping the bulk fluidhydrodynamic forces, shear and residence times low. All edges and thejoins between the blades and the support cone should be smoothed.

The means of providing the driving torque on the impeller 100 of thepreferred embodiment of the invention is to encapsulate permanentmagnets 14 in the blades 8 of the impeller 100 and to drive them with arotating magnetic field pattern from oscillating currents in windings 15and 16, fixed relative to the housing 2. Magnets of high remanence suchas sintered rare-earth magnets should be used to maximise motorefficiency. The magnets should be aligned axially or approximatelyaxially, with alternating polarity for adjacent blades. Thus there mustbe an even number of blades. Since low blade number is preferred for thebearing force, and since two blades would not have sufficient bearingstiffness to rotation about an axis through the blades and perpendicularto the pump housing (unless the blades are very curved), four blades arerecommended. A higher number of blades, for example 6 or 8 will alsowork.

Some possible options for locating the magnets 14 within the blades 8are shown in FIG. 4. The most preferred which is depicted in FIG. 4A, isfor the blade to be made of magnet material apart from a biocompatibleshell or coating to prevent fluid corroding the magnets and to preventmagnet material (which may be toxic) entering the blood stream. Thecoating should also be sufficiently durable especially at blade cornersto withstand rubbing during start-up or during inadvertent bearing touchdown.

In one particular form the inside walls of the pump housing 2 are alsocoated with a biologically compatible and wear resistant material suchas diamond coating or titanium nitride so that wear on both of thetouching surfaces is minimised.

An acceptable coating thickness is approximately 1 micron.

A suitable impeller manufacturing method is to die-press the entireimpeller, blades and support cone, as a single axially aligned magnet.The die-pressing is much simplified if near axially uniform blades areused (blades with an overhang such as in FIG. 3C are precluded). Duringpressing, the crushed rare-earth particles must be aligned in an axialmagnetic field. This method of die-pressing with parallel alignmentdirection is cheaper for rare-earth magnets, although it producesslightly lower remanence magnets. The tolerance in die-pressing is poor,and grinding of the tapered blade edges is required. Then the magnetimpeller can be coated, for example by physical vapour deposition, oftitanium nitride for example, or by chemical vapour deposition, of athin diamond coating or a teflon coating.

In an alternative form the magnet material can be potted in titanium ora polymeric housing which is then, in turn, coated with a biologicallycompatible and tough material such as diamond coating or titaniumnitride.

Finally, to create the alternating blade polarity the impeller must beplaced in a special pulse magnetisation fixture, with an individual coilsurrounding each blade. The support cone may acquire some magnetisationnear the blades, with negligible influence.

Alternative magnet locations are sketched in FIG. 4B and FIG. 4C inwhich quadrilateral or circular cross-section magnets 14 are insertedinto the blades. Sealing and smoothing of the blade edges over theinsertion holes is then required to reinstate the taper.

All edges in the pump should be radiused and surfaces smoothed to avoidpossible damage to formed elements of the blood.

The windings 15 and 16 of the preferred embodiment are slotless orair-gap windings, following the blade curvature, with the. same polenumber as the impeller, namely four poles in the preferred embodiment. Aferromagnetic iron yoke 17 of conical form for the front winding and aniron ferromagnetic yoke 18 of annular form for the back winding may beplaced on the outside of the windings to increase the magnetic fluxdensities and hence increase motor efficiency. The winding thicknessesshould be designed for maximum motor efficiency, with the sum of theiraxial thicknesses somewhat less than but comparable to the magnet axiallength. The yokes can be made of solid ferromagnetic material such asiron. To reduce “iron” losses, the yokes 17 can be laminated, forexample by helically winding thin strip, or can be made of iron/powderepoxy composite. Alternatively they can be helically wound to reduceiron losses. The yokes should be positioned such that there is zero netaxial magnetic force on the impeller when it is positioned centrally inthe housing. The magnetic force is unstable and increases linearly withaxial displacement of the impeller away from the central position, withthe gradient being called the positive stiffness of the magnetic force.This unstable magnetic force must be countered by the hydrodynamicbearings, and so the stiffness should be made as small as possible.Choosing the yoke thickness such that the flux density is at thesaturation level reduces the stiffness and gives minimum mass. Analternative would be to have no iron yokes, completely eliminating theunstable axial magnetic force, but the efficiency of such designs wouldbe lower and the magnetic flux density in the immediate vicinity of thepump may violate safety standards and produce some tissue heating. Inany case, the stiffness is acceptably small for slotless windings withthe yokes present. Another alternative would be to insert the windingsin slots in laminated iron stators which would increase motor efficiencyand enable use of less magnet material and potentially lighter impellerblades. However, the unstable magnetic forces would be significant forsuch slotted motors. Also, the necessity for fat blades to generate therequired bearing forces allows room for large magnets, and so slotlesswindings are chosen in the preferred embodiment.

FIG. 5 depicts one suitable topology for the front face winding 15. Theback face winding 16 looks similar from the back end of the motor,except the hole on the axis is smaller. Each winding has three phases,A, B and C, and two coils connected in series or parallel per phase.Each coil comprises a number of turns of an insulated conductor such ascopper, with the number of turns chosen to suit the desired voltage. Theconductor may need to be stranded to reduce eddy losses. The windingconstruction can be simplified by laying the coils around pinsprotruding from a temporary conical former, the pins shown as dots intwo rings of six pins each in FIG. 5. The coils are labelledalphabetically in the order in which they would be layed, coils a and dfor phase A, b and e for phase B, and c and f for phase C. Instead of oras well as pins, the coil locations can be defined by thin curved fins,running between the pins in FIG. 5, along the boundary between thecoils.

The winding connection of the preferred embodiment is for three wires,one wire per phase, to connect a sensorless electronic controller towinding 15, three wires to pass between windings 15 and 16, and for aneutral point termination of the wires within winding 16. A neutrallead, N in FIG. 5, between the controller and the neutral point isoptional. A standard sensorless controller can be used, in which two outof six semiconducting switches in a three phase bridge are turned on atany one time, with the switching synchronised with the impeller positionvia the back-emf in the unenergised phase. Alternatively, because of therelatively small fraction of the impeller cross-section occupied bymagnets, it may be slightly more efficient to only activate one of thethree phases at a time, and to return the current by a fourth wire fromthe winding 16 neutral point back to the controller. The provision ofthe neutral lead also enables redundancy to be built into the motor andcontroller, so that if any one of the three phases fails in either themotor or controller, then the other two phases can still provide arotating magnetic field sufficient to drive the pump. Careful attentionmust be paid to ensure that the integrity of all leads and connectionsis failsafe.

In the preferred embodiment, the two housing components 3 and 4 are madeby injection moulding from non-conducting plastIc materials such asLexan polycarbonate plastic or ceramics. The windings and yokes areencapsulated within the housing during fabrication moulding. In thisway, the separation between the winding and the magnets is minimised,increasing the motor efficiency, and the housing is thick, increasingits mechanical stiffness. Alternatively, the windings can be positionedoutside the housing, of thickness at least around 2 mm for sufficientstiffness.

If the housing material plastic is hygroscopic or if the windings areoutside the housing, it may be necessary to first enclose the windingsand yoke in a very thin impermeable shell. Ideally the shell should benon-conducting (such as ceramic or plastic), but titanium of around 0.1mm to 0.2 mm thickness would give sufficiently low eddy losses.Encapsulation within such a shell would be needed to prevent windingmovement.

By keeping the windings separate for the front and back faces, thewindings can be moulded into the front and back housing parts.Alternatively, for the case of windings not moulded into the housings,it may be possible to wind the coils onto the assembled housing, passingthe coils from the front face to the back face over the volute 13.

The combining of the motor and bearing components into the impeller inthe preferred embodiment provides several key advantages. The rotorconsequently has very simple form, with the only cost of the bearingbeing tight manufacturing tolerances. The rotor mass is very low,minimising the bearing force needed to overcome weight. Also, with thebearings and the motor in the same region of the rotor, the bearingsforces are smaller than if they had to provide a torque to supportmagnets at an extremity of the rotor.

A disadvantage of the combination of functions in the impeller is thatits design is a coupled problem. The optimisation should ideally linkthe fluid dynamics, magnetics and bearing thrust calculations. Inreality, the blade thickness can be first roughly sized to give adequatemotor efficiency and sufficient bearing forces with a safety margin.Fortuitously, both requirements are met for four blades of approximateaverage circumferential thickness 5 mm. The housing, blade, and supportcone shapes can then be designed using computational fluid dynamics,maintaining the above minimum average blade thickness. Finally the motorstator, i.e. winding and yoke, can be optimised for maximum motorefficiency.

FIG. 6 depicts an alternative embodiment of the invention as an axialpump. The pump housing is made of two parts, a front part 19 and a backpart 20, joined for example at 21. The pump has an axial inlet 22 andaxial outlet 23. The impeller comprises only blades 24 mounted on asupport cylinder 25 of reducing radius at each end. An important featureof this embodiment is that the blade edges are tapered to generatehydrodynamic thrust forces which suspend the impeller. These forcescould be used for radial suspension alone from the straight section 26of the housing, with some alternative means used for axial suspension,such as stable axial magnetic forces or a conventional tapered-land typehydrodynamic thrust bearing. FIG. 6 proposes a design which uses thetapered blade edges to also provide an axial hydrodynamic bearing. Thehousing is made with a reducing radius at its ends to form a front face27 and a back face 28 from which the axial thrusts can suspend the motoraxially. Magnets are embedded in the blades with blades havingalternating polarity and four blades being recommended. Iron in theouter radius of the support cylinder 25 can be used to increase themagnet flux density. Alternatively, the magnets could be housed in thesupport cylinder and iron could be used in the blades. A slotlesshelical winding 29 is recommended, with outward bending end-windings 30at one end to enable insertion of the impeller and inward bendingwindings 31 at the other end to enable insertion of the winding into acylindrical magnetic yoke 32. The winding can be encapsulated in theback housing part 20.

Third Embodiment

With reference to FIGS. 7 to 15 inclusive there is shown a furtherpreferred embodiment of the pump assembly 200.

With particular reference initially to FIG. 7 the pump assembly 200comprises a housing body 201 adapted for bolted connection to a housingcover 202 and so as to define a centrifugal pump cavity 203 therewithin.

The cavity 203 houses an impeller 204 adapted to receive magnets 205within cavities 206 defined within blades 207. As for the firstembodiment the blades 207 are supported from a support cone 208.

Exterior to the cavity 203 but forming part of the pump assembly 200there is located a body winding 209 symmetrically mounted around inlet210 and housed between the housing body 201 and a body yoke 211.

Also forming part of the pump assembly 200 and also mounted external topump cavity 203 is cover winding 212 located within winding cavity 213which, in turn, is located within housing cover 202 and closed by coveryoke 214.

The windings 212 and 209 are supplied from the electronic controller ofFIG. 12. As for the first embodiment the windings are arranged toreceive a three phase electrical supply and so as to set up a rotatingelectrical field within cavity 203 which exerts a torque on magnets 205within the impeller 204 so as to urge the impeller 204 to rotatesubstantially about central axis TT of cavity 203 and in line with thelongitudinal axis of inlet 210. The impeller 204 is caused to rotate soas to urge fluid (in this case blood liquid) around volute 215 andthrough outlet 216.

The assembly is bolted together in the manner indicated by screws 217.The yokes 211, 214 are held in place by fasteners 218. Alternatively,press fitting is possible provided sufficient integrity of seal can bemaintained.

FIG. 8 shows the impeller 204 of this embodiment and clearly shows thesupport cone 208 from which the blades 207 extend. The axial cavity 219which is arranged, in use, to be aligned with the longitudinal axis ofinlet 210 and through which blood is received for urging by blades 207is clearly visible.

The cutaway view of FIG. 9 shows the axial cavity 219 and also themagnet cavities 206 located within each blade 207. The preferred conestructure 220 extending from housing cover 202 aligned with the axis ofinlet 210 and axial cavity 219 of impeller 204 is also shown.

FIG. 10 is a side section, indicative view of the impeller 204 definingthe orientations of central axis FF, top taper edge DD and bottom taperedge BB, which tapers are illustrated in FIG. 11 in side section view.

FIG. 11A is a section of a blade 207 of impeller 204 taken through planeDD as defined in FIG. 10 and shows the top edge 221 to be profiled froma leading edge 223 to a trailing edge 224 as follows: central portion227 comprises an ellipse having a semi-major axis of radius 113 mm and asemi-minor axis of radius 80 mm subtended on either side by a region ofno radius and then followed by leading conical surface 225 and trailingconical surface 226 on either side thereof as illustrated in FIG. 11A.

The leading edge 223 is radiused as illustrated.

FIG. 11B illustrates in cross-section the bottom edge 222 of blade 207cut along plane BB of FIG. 10.

The bottom edge includes cap 228 utilised for sealing magnet 205 withincavity 206.

In this instance substantially the entire edge comprises a straighttaper with a radius of 0.05 mm at leading edge 229 and a radius of 0.25mm at trailing edge 230.

The blade 207 is 5.4 mm in width excluding the radii at either end.

FIG. 12 comprises a block diagram of the electrical controller suitablefor driving the pump assembly 200 and comprises a three phasecommutation controller 232 adapted to drive the windings 209, 212 of thepump assembly. The commutation controller 232 determines relative phaseand frequency values for driving the windings with reference to setpoint speed input 233 derived from physiological controller 234 which,in turn, receives control inputs 235 comprising motor current input andmotor speed (derived from the commutation controller 232), patient bloodflow 236, and venous oxygen saturation 237.

FIG. 13 is a graph of pressure against flow for the pump assembly 200where the fluid pumped is 18% glycerol for impeller rotation velocityover the range 1500 RPM to 2500 RPM. The 18% glycerol liquid is believedto be a good analogue for blood under certain circumstances.

FIG. 14 graphs pump efficiency against flow for the same fluid over thesame speed ranges as for FIG. 13.

FIG. 15 is a graph of electrical power consumption against flow for thesame fluid over the same speed ranges as for FIG. 13.

FURTHER EMBODIMENTS

The common theme running through the first, second and third embodimentsdescribed thus far is the inclusion in the impeller of a taper or otherdeformed surface which, in use, moves relative to the adjacent housingwall thereby to cause a restriction with respect to the line of movementof the taper or deformity thereby to generate thrust upon the impellerwhich includes a component substantially normal to the line of movementof the surface and also normal to the adjacent internal pump wall withrespect to which the restriction is defined for fluid locatedtherebetween.

In order to provide both radial and axial direction control at least oneset of surfaces must be angled with respect to the longitudinal axis ofthe impeller (preferably at approximately 45° thereto) thereby togenerate or resolve opposed radial forces and an axial force which canbe balanced by a corresponding axial force generated by at least oneother tapered or deformed surface located elsewhere on the impeller.

In the forms thus far described top surfaces of the blades 8, 207 areangled at approximately 450 with respect to the longitudinal axis of theimpeller 100, 204 and arranged for rotation with respect to the internalwalls of a similarly angled conical pump housing. The top surfaces aredeformed so as to create the necessary restriction in the gap betweenthe top surfaces of the blades and the internal walls of the conicalpump housing thereby to generate a thrust which can be resolved to bothradial and axial components.

In the examples thus far the bottom faces of the blades 8, 207 comprisesurfaces substantially lying in a plane at right angles to the axis ofrotation of the impeller and, with their deformities define a gap withrespect to a lower inside face of the pump housing against which asubstantially only axial thrust is generated.

Other arrangements are possible which will also, relying on theseprincipals, provide the necessary balanced radial and axial forces. Sucharrangements can include a double cone arrangement where the conical topsurface of the blades is mirrored in a corresponding bottom conicalsurface. The only concern with this arrangement is the increased depthof pump which can be a problem for in vivo applications where sizeminimisation is an important criteria.

With reference to FIG. 18 a further embodiment of the invention isillustrated comprising a plan view of the impeller 300 forming part of a“channel” pump. In this embodiment the blades 301 have been widenedrelative to the blades 207 of the third embodiment to the point wherethey are almost sector-shaped and the flow gaps between adjacent blades301, as a result, take the form of a channel 302, all in communicationwith axial cavity 303.

A further modification of this arrangement is illustrated in FIG. 19wherein impeller 304 includes sector-shaped blades 305 having curvedleading and trailing portions 306, 307 respectively thereby definingchannels 308 having fluted exit portions 309.

As with the first and second embodiments the radial and axialhydrodynamic forces are generated by appropriate profiling of the topand bottom faces of the blades 301, 305 (not shown in FIGS. 18 and 19).

A further embodiment of a pump assembly according to the inventioncomprises an impeller 310 as illustrated in FIG. 20 where, conceptually,the upper and lower surfaces of the blades of previous embodiments areinterconnected by a top shroud 311 and a bottom shroud 312. In thisembodiment the blades 313 can be reduced to a very small width as thehydrodynamic behaviour imparted by their surfaces in previousembodiments is now given effect by the profiling of the shrouds 311, 312which, in this instance, comprises a series of smooth-edged wedges withthe leading surface of one wedge directly interconnected to the trailingedge of the next leading wedge 314.

As for previous embodiments the top shroud 311 is of overall conicalshape thereby to impart both radial and axial thrust forces whilst thebottom shroud 312 is substantially planar thereby to impartsubstantially only axial thrust forces.

The foregoing describes the principles of the present invention, andmodifications, obvious to those skilled in the art, can be made theretowithout departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

The pump assembly 1, 200 is applicable to pump fluids such as blood on acontinuous basis. With its expected reliability it is particularlyapplicable as an in vivo heart assist pump.

The pump assembly can also be used with advantage for the pumping ofother fluids where damage to the fluid due to high shear stresses mustbe avoided or where leakage of the fluid must be prevented with a veryhigh degree of reliability—for example where the fluid is a dangerousfluid.

1. (canceled)
 2. A rotary blood pump for use in a heart assist device, said pump having an impeller suspended within a pump housing by hydrodynamic thrust forces generated by relative movement of said impeller with respect to and within said pump housing; and wherein at least one of said impeller and said housing includes at least a first deformed surface lying on at least part of a first face which, in use, moves relative to respective facing surfaces on the other of said impeller and said housing thereby to form a relatively moving surface pair which generates relative hydrodynamic thrust between said impeller and said housing which includes everywhere a localized thrust component substantially and everywhere normal to said first deformed surface.
 3. The pump of claim 2, wherein the pump includes a hydrodynamic bearing.
 4. The pump of claim 2, wherein said pump includes radial and axial direction control and said radial and axial direction control is provided by the inclusion of one set of surfaces on said pump angled with respect to the rotational axis of the impeller.
 5. The pump of claim 2, wherein said impeller includes blades which are tapered or non-planar, so that a thrust is created between edges of said blades and the pump housing during relative movement therebetween.
 6. The blood pump of claim 2 wherein said pump is of centrifugal type or mixed flow configuration, wherein said impeller includes blades which form gaps in said impeller, and wherein said gaps open on both front and back faces of the impeller.
 7. The pump of claim 6, wherein the front face of the pump housing is generally conical shaped, to allow said thrust forces to be generated generally perpendicular to an inner conical surface of the housing and wherein said thrust forces have a radial component to resist radial displacement of the impeller axis.
 8. The pump of claim 2, wherein said first deformed surface and is integral to at least one surface of said impeller.
 9. A rotary blood pump for assisting blood circulation comprising: a plastic, metal or ceramic housing; a hydrodynamically suspended impeller wherein said impeller, in use, is magnetically urged to rotate; and at least one stator assembly.
 10. The pump of claim 9, wherein at least a portion of said pump is coated with a biocompatible film.
 11. The pump of claim 10, wherein said film includes titanium nitride or carbon.
 12. The pump of claim 9, wherein said impeller includes at least one blade.
 13. An implantable rotary blood pump comprising: a housing; an impeller where said impeller, in use, is magnetically urged to rotate; a hydrodynamic bearing formed by the cooperation of a surface of said impeller and said housing; and at least one stator assembly.
 14. A rotary blood pump comprising: a housing, at least one stator assembly, and a hydrodynamically suspended impeller; wherein said impeller carries at least one permanent magnet; and wherein said magnet produces an axis of magnetism and wherein said axis of magnetism is offset at an angle extending away from the axis of rotation of said impeller.
 15. A rotary blood pump comprising: a housing; a hydrodynamically suspended impeller wherein said impeller, in use, is magnetically urged to rotate; at least one stator assembly and wherein said stator assembly and impeller cooperate to form a three phase motor.
 16. A rotary blood pump comprising: a housing, a hydrodynamically suspended impeller wherein said impeller, in use, is magnetically urged to rotate, at least two stator assemblies wherein at least a portion of one said stator assembly is aligned at generally 45° to an axis of rotation of the said impeller.
 17. A rotary blood pump comprising: a housing; a hydrodynamically suspended impeller wherein said impeller is magnetically urged to rotate; and at least one stator assembly.
 18. The pump of claim 17 wherein said housing is polymeric.
 19. The pump of claim 17 wherein movement of said impeller generates axial and/or radial thrust forces.
 20. The pump of claim 17 wherein said impeller includes an integral hydrodynamic bearing surface.
 21. The pump of claim 17, wherein said pump is shaftless.
 22. The pump of claim 21, wherein said impeller includes at least two blades.
 23. The pump of claim 22, wherein said blades are supported by at least one support cone.
 24. The pump of claim 23, wherein said support cone generates a partial bearing means by the generation of hydrodynamic thrust forces. 